1. Field of the Invention
The present invention relates to a method of driving a display element that uses cholesteric liquid crystals, and more particularly, to a method of driving a liquid crystal display element by which a partial screen can be rewritten at high speed.
2. Description of the Related Art
In recent years, electronic paper has been vigorously developed by companies and universities. Electronic paper can be applied to various portable devices including electronic books, sub-displays in mobile terminals, and display units in IC cards.
One effective way to realize electronic paper is to utilize cholesteric liquid crystals.
A cholesteric liquid crystal has excellent characteristics, including an ability to hold a display state semi-permanently (image memory characteristic) and to display images clearly in full color at a high contrast and at a high resolution. The cholesteric liquid crystal is also called a chiral nematic liquid crystal because the cholesteric liquid crystal is a nematic liquid crystal whose cholesteric phase is formed, and the cholesteric phase where molecules of the nematic liquid crystal are tied up in a helix is formed by adding a relatively large quantity (several tens of percent) of chiral addition (also called chiral material) to the nematic liquid crystal.
Hereinafter, the principles of the display and of the driving of cholesteric liquid crystals are explained.
A display using cholesteric liquid crystals is controlled in accordance with the oriented state of the molecules in the cholesteric liquid crystals. As shown in the graph of a reflection factor in FIG. 1A, cholesteric liquid crystals have a planar (P) state, where the incident light is reflected, and a focal conic (FC) state, where the incident light penetrates, and these states are stable even without an electric field. In the planar state, light having a wavelength corresponding to the helical pitch over the liquid crystal molecules is reflected. The wavelength λ that causes the maximum reflection is expressed by the equation below in which n is an average refraction index, and p is a helical pitch.λ=n·p 
In contrast, the reflection band Δλ increases as the refraction index anisotropy Δn increases.
Accordingly, by suitably selecting the average refraction index n and helical pitch p, it is possible to display a color having the wavelength λ, in the planar state.
Also, by providing a light absorption layer separately from a liquid crystal layer, black can be displayed in the focal conic state.
Next, an example of driving cholesteric liquid crystals is explained.
When an intense electric field is applied to a cholesteric liquid crystal, the helical structure of the liquid crystal molecules are unwound completely and their state becomes homeotropic, with all the molecules oriented along the direction of the electric field. Next, when the electric field that has caused the homeotropic state suddenly becomes zero, the helical axis of the liquid crystal becomes perpendicular to the electrode, and the planar state is caused in which light is selectively reflected in accordance with the helical pitch. In contrast, when an electric field that is sufficiently weak so as not to unwind the helical structure is removed, or when an intense electric field is gradually removed after being applied, the helical axis of the liquid crystal becomes parallel to the electrode, and the focal conic state is caused in which the incident light penetrates. Also, when an intermediately intense electric field is applied and this electric field is removed suddenly, both the planar state and the focal conic state are caused and a display of halftones is possible.
By using this phenomenon, information can be displayed.
The above described voltage response characteristic is summarized as follows with reference to FIG. 1 explaining the response characteristic of the cholesteric liquid crystal.
If the initial state (0V) is the planar state (high reflectance), a driving voltage value enters a driving band for the focal conic state (low reflectance) when the pulse voltage is raised to a certain range (such as 24V), and the driving voltage value reenters a driving band for the planar state (high reflectance) when the pulse voltage is further raised (such as to 32V). If the initial state (0V) is the focal conic state, the driving voltage value gradually enters the driving band for the planar state as the pulse voltage is raised.
Here, voltage outputs of a marketed STN driver of two-valued output, which is applicable to cholestric liquid crystal, are described with reference to FIGS. 2A to 2C.
Since alternating current driving is essential for liquid crystal, two driving voltages used when an alternating current signal is High and Low must be initially set. Correspondences between the voltages (V0, V21, V34, V5) and the alternating current signal High or Low at that time are normally as shown in FIG. 2A for a data signal and a scan signal, and the magnitudes of the voltage outputs must be V0≧V21≧V34≧V5.
By way of example, for data fed in a segment mode, a voltage output is V0 if a data signal is High and its corresponding alternating current signal is High, or a voltage output is V5 if the data signal is High and its corresponding alternating current signal is Low, as shown in FIG. 2A.
In accordance with this, when cholestreric liquid crystal is driven, for example, in the segment mode, the voltages (V0, V21, V34, V5) are set to voltage values of (32, 24, 8, 0) respectively. Then, voltage waveforms shown in FIG. 2C are obtained.
Namely, to a line selected by scanning, a voltage 0→32V is applied like “ON scan” shown in FIG. 2C. In the meantime, a voltage 32→0V and a voltage 24→8V are applied respectively as ON data and OFF data from the data side. These voltages are combined, whereby, in the selected line, a potential difference of “ON scan-ON data” occurs in a pixel of ON display and a voltage of ±32V is applied, and a potential difference of “ON scan-OFF data” occurs in a pixel of OFF display, and a voltage of ±24 is applied.
Similarly, for all of pixels in a nonselected line, potential differences of “OFF scan-ON data”, and “OFF scan-OFF data” occur, and ±4V is applied to all of them.
Accordingly, as understood from the voltage response characteristic shown in FIG. 1, crosstalk does not occur in a nonselected line, and a preceding display state (the planar state or the focal conic state) is held.
Incidentally, for electronic paper, which is an application field of a display element using cholesteric liquid crystal, a function to rewrite only a particular region within a display area (hereinafter referred to as a partial rewrite) is demanded. When a partial rewrite is made, other regions must hold a previously written state unchanged.
However, a marketed STN driver has the following problems. (1) Since the scan mode shifts all of lines without fail, ON/OFF voltage is applied also to a region, to which a partial rewrite is not made, in all cases. Namely, scanning cannot be made only for a particular region. Since all of lines are scanned after all, this poses a problem that a lot of time is required because an entire screen is rewritten by combining displayed image data and image data used for the partial rewrite in a processing unit. (2) Only a particular line can be scanned if both a scan side and a data side are driven in the segment mode. In that case, a largeness/smallness relationship of voltages like V0≧V21≧V34≧V5 becomes a barrier, and a driving waveform obtained by combining the common mode and the segment mode cannot be realized, and crosstalk occurs in a nonselected pixel in all cases, so that a displayed image is lost.
In the meantime, the known partial rewrite methods for cholesteric liquid crystal recited in the following Patent Documents 1 to 5 use a customized driver the voltage output of which is made bipolar (plus and minus) and which is free from the above described voltage constraints.
Here, examples where the cholesteric liquid crystal is simply driven in accordance with the response characteristic of the liquid crystal shown in FIG. 1 tentatively with the bipolar driver free from the voltage constraints are shown in FIG. 3. In both of the examples 1 and 2, voltages of ON level and OFF level, which are shown in FIG. 2C, can be applied to pixels in a selected line. As described above, it becomes easy to select and drive only a partial rewrite region if the voltage constraints are not imposed.
However, since the driver is made bipolar, high voltage endurance and a complicated configuration of an LSI within the driver are required, leading to an increase in cost.
Namely, to partially rewrite a particular region of a display element using cholesteric liquid crystal, conventionally, both a scan driver and a data driver must drive in a mode (usually called a segment mode) for enabling the selection of an arbitrary line, and must be designed to have a special output voltage in order to prevent crosstalk. This is a large factor to increase the cost. With a universal STN driver, a partial rewrite cannot be made because a large amount of crosstalk occurs in the segment mode of both the scan driver and the data driver as described above. Even if part of a display is rewritten, an entire image must be updated. Accordingly, there is problem that a rewrite requires a lot of time.
Patent Document 1: Japanese Patent Application Publication No. 2000-147465
Patent Document 2: Japanese Patent Application Publication No. 2000-147466
Patent Document 3: Japanese Patent Application Publication No. 2000-171837
Patent Document 4: Japanese Patent Application Publication No. 2000-180887
Patent Document 5: Japanese Patent Application Publication No. 2000-194005